Experiments will be carried out to elucidate the effect of the intake of dietary phosphate and calcium, the Ca:P ratio and the level of dietary protein on the development of osteoporosis in aging rats and mice. The influence of high Ca and high protein diets, and of forced exercise, on the loss of bony tissue induced by high phosphate intakes will be evaluated, and the relationship between dietary P and parathyroid hormone synthesis will be determined. The experiments will shed light on factors of possible importance in the development of osteoporosis in the adult human, including the influence of a high phosphate intake, the moderating effect of a high Ca intake on the metabolic response to phosphate, and the consequences of decreasing physical activity.